Ultrasonic characterization of cracks in materials is at least a two-step process: 1) detection and location; and 2) sizing in absolute or relative terms.
U.S. Pat. No. 5,118,464 discloses a method for detecting the presence of material flaws, such as a crack 8 (see FIG. 1 herein), behind normally opaque barriers, such as fluid-filled gap 6 between structures 2 and 4. In this method transducer 10 is excited to emit a longitudinal ultrasonic wave which is coupled to structure 2 via couplant 12, e.g., normal water in a nuclear reactor. At the correct frequency the emitted wave bridges the gap and enters structure 4, where it is reflected by crack 8. The return path of the reflected wave also bridges the gap and the wave impinges on the transducer 10, where it is detected as a "pulse echo" signal.
However, the determination of the crack size, or depth of penetration in the case of surface-connected flaws, is a different and more complicated task. Because of the special constraints placed on the system by the presence of the fluid-filled gap, novel means are required to determine the crack size.
A conventional method for determining the depth of penetration of a planar crack 8', known as the time-of-flight diffraction technique, is illustrated in FIG. 2. This method takes advantage of the forward scattering of waves of ultrasonic energy at the edge 24 of crack 8' connected to back surface 18 of wall 14. An emitter of short pulses of ultrasound, coupled to the inspection surface 16 at location 20 which is a distance S from the plane 26 of the crack (or its vertical projection), causes refracted sound waves 22 to impinge on the crack edge 24, which scatters the ultrasonic energy in all directions. A detector situated at location 30, a distance S on the opposite side of the crack plane 26, is excited by the ray 28 of scattered pulsed energy after a time delay that is a function of S, the crack height h, and the known wall thickness D.sub.w.
In accordance with this method, the two legs of the detection triangle need not be equal in length, but the geometry is simpler for this case, and the same type of transducer can be used for emission and detection. Either shear or longitudinal waves may be used, depending on the type of transducer employed.
By measuring the time-of-flight of the pulses from the emitter to the detector by way of the crack edge, the crack height h can be easily computed from the geometry of FIG. 2 (although the time-of-flight does not determine the details of crack orientation, or aspect).
Inspection methods using the ultrasonic time-of-flight diffraction technique have been devised for buried, as well as surface-connected, cracks and have proven to be the most accurate means of crack sizing in practice. Corrections for surface curvature effects can be employed for use with pipes and nozzles, where necessary, to enhance accuracy. On the other hand, the method clearly fails if the scattered wave is unable to reach the detector, which occurs if there is a relatively non-uniform gap interposed between the inspection surface 16 and the crack edge 24.
Detection and sizing of cracks behind machined gaps are required for inspection of certain structures in nuclear power plants. These gaps are present by design and usually are not entirely uniform, because of dimensional tolerances and variations in positioning and joining. In cases of interest, the gap may have water or helium gas injected to assure that the resonant frequency is consistent with ultrasonic frequencies commonly used for detection and sizing of material cracks. However, for non-uniform gaps, the gap transmission frequency varies along the gap and is a weak function of the angle-of-incidence at the gap surface, as will be shown below. This makes the standard time-of-flight method inapplicable, since little scattered energy reaches the detector location shown in FIG. 2.